1. Field of the Invention
The present invention relates to a power converter for driving a motor at a variable speed, having a rectifier connected to an AC power source and a voltage type inverter for inverting a DC voltage, which is supplied through a DC intermediate circuit including a smoothing capacitor from the rectifier, produce an AC voltage having a variable frequency and a variable voltage which is supplied to an AC motor.
2. Description of the Prior Art
With the power converter of the type described above, the AC motor is pulled into power generation operation under the control of the voltage type inverter to perform braking operation. In this case, the kinetic energy of the motor and a load is converted into an electrical energy by the motor, and the electrical energy is returned via the inverter to the DC intermediate circuit. Therefore, in order to limit a voltage rise across the smoothing capacitor in the DC intermediate circuit within an allowable range, it is required to Process the electrical energy.
In order, to process the electrical energy efficiently, while avoiding an undesired consumption of the power, it is well known in the art that the electric energy returned to the DC intermediate circuit is regenerated into the AC power source during the braking operation of the motor. In this case, if a thyristor converter is provided as a rectifier for feeding an electric power to the DC intermediate circuit during the driving operation of the motor, and the thyristor converter is used for the regeneration during braking operation of the motor, another thyristor converter exclusively for the sake of the regeneration can be eliminated. In this case, in order to ensure that the voltage polarities of the smoothing capacitor remain unchanged even during the driving operation and the regenerative braking operation of the motor, there is provided means for switching the polarities of the connection between the smoothing capacitor and the thyristor converter.
FIG. 9 shows a conventional power converter for driving a motor at a variable speed, provided with the switching means of the type described above.
In this power converter, the DC terminals of a thyristor converter 11, which are connected to three phase lines R, S and T of a three phase AC power source, are connected through diodes 13 and 15 to a smoothing capacitor 17, respectively. A voltage type inverter 18 connected to the smoothing capacitor 17 is formed by a three phase bridge connection of transistors to which diodes are connected in parallel, respectively, to supply an AC voltage of a variable frequency and a variable voltage to an AC motor 19. Furthermore, the thyristor converter 11 and the smoothing capacitor 17 are connected through a series circuit of a transistor 14 and a resistor 21 on one hand and through a series circuit of a transistor 16 and a resistor 22 on the other hand in such a way that the thyristor converter 11 and the smoothing capacitor 17 are connected to each other in reversed polarity with respect to the connections of the diodes 13 and 15.
With this arrangement, in the case of driving operation, the electric power is fed from the AC power source through the thyristor converter 11 and the diodes 13 and 15 to the smoothing capacitor 17 by the forward conversion operation accomplished by the thyristor 11 and then further fed to the AC motor 10 from the smoothing capacitor 17 through the inverter 18.
In the case of braking operation, the electric energy is returned to the smoothing capacitor 17 from the AC motor 19 through the inverter 18 and excess energy stored in the smoothing capacitor 17 is regenerated to the AC power source through the resistors 21 and 22, transistors 14 and 16 and the thyristor converter 11 by the reverse conversion operation of the thyristor converter 11.
In this conventional converter, the transistors 14 and 16 are maintained in the conduction state in the case of braking operation, and the regenerative current flowing through the transistors 14 and 16 is adjusted by the phase control by the thyristor converter 11. In the case of the driving operation, in order to attain a high voltage availability of the thyristor converter 11, the forward conversion operation of the thyristor converter 11 is carried out at a firing angle of lag substantially equal to zero, so that the same maximum output voltage can be produced as that in the case of a diode rectifier. On the other hand, in the reverse conversion operation of the thyristor converter 11 in the case of the braking operation, in order to avoid commutation failure, a suitable commutation margin angle must be secured. For this purpose, a maximum possible reverse voltage obtained across the DC terminals of the thyristor converter 11 is lower than the maximum output voltage obtained in the above-mentioned forward conversion operation. Therefore, in the case of the braking operation, the maximum possible reverse voltage from the thyristor 11 is lower than the voltage across the smoothing capacitor 16. The regenerative current flowing through the transistors 14 and 16 would be out of control and thus dissipated without the provision of the resistor 21 and 22, so that the transistors 14 and 16 are protected against an overcurrent.
The resistance R (.OMEGA.) of each resistor is determined as follows. It is assumed that E.sub.1 (V) represents the assumed minimum value of the DC reverse voltage generated by the thyristor converter 11 when the thyristor converter 11 is driven at a predetermined minimum control angle of lead under the consideration of the variations of the power source voltage and it is assumed that E.sub.2 (V) represents the assumed maximum value of the voltage across the smoothing capacitor 17. Then, the resistance R (.OMEGA.) of the resistor is so determined that the current value (A) given by (E.sub.2 -E.sub.1)/2R.OMEGA. remains within a tolerable range for each of the transistors 14 and 16.
In the converter of the type described above, a power loss of i.sup.2 R(W) (where i represents a generative current [A] flowing through the resistors 21 and 22) is generated in each of the resistors 21 and 22 so that heat is generated. Thus, in the case of the design and production of the power converter for driving a motor at a variable speed, there arises a serious problem in that it is difficult to dissipate the heat generated by the resistors 21 and 22. Because of this problem, it has been difficult to make a power converter compact in size, light in weight and inexpensive to manufacture.